Later this month Japanese politicians will be fighting an election that will decide the make-up of the upper house of parliament.

It is a first important electoral test for Shinzo Abe as prime minister. He took on the job last autumn and his popularity has been in decline ever since.

One issue that is attracting a fair amount of attention is his plan to reform the country's pacifist constitution.

The document was written by the Americans more than 60 years ago when they occupied Japan after the end of World War II.

They arrived in Japan with new ideas like democracy and women's rights, which they imposed in the new constitution which was designed to ensure that Japan would never go to war again.

'Ambiguous'

The document was first written in English and then translated into Japanese. That proved to be significant.

It meant that it could be read in different ways depending on your point of view - one reason, says Professor Phil Deans from Temple University in Tokyo, why it has not been rewritten in 60 years.

It's all done by interpretation so I think it is very ambiguous

Taro KonoLDP lawmaker

"It was translated into Japanese by Japanese bureaucrats who left a certain degree of room for manoeuvre," he says.

"They were able to leave gaps in the language which subsequently politicians have been able to reinterpret, and so instead of constitutional revision per se, what you have is revision through reinterpretation."

The constitution contains 103 articles or clauses. In Article 9 Japan promises never again to fight other nations, and what is more, never to maintain land, sea or armed forces capable of waging war.

"The wording of Article 9 has not changed for the last 60 years," points out Taro Kono, an MP from the Liberal Democratic Party (LDP) which has governed Japan almost without a break for that whole period.

"The first prime minister under this constitution, Mr Yoshida, who was a conservative politician, actually said in parliament that this constitution doesn't allow the Japanese government to have any soldiers.

"And yet the same wording was used by [former] Prime Minister Junichiro Koizumi to send the soldiers to Iraq."

"It's all done by interpretation so I think it is very ambiguous," he complains.

'An embarrassment'

Japan, of course, has built up a sizeable body of fighting men and women over the last six decades, known euphemistically as the Self-Defence Forces.

Mr Abe has made revising the constitution his priority

It is only relatively recently, though, that the LDP has had a leader from the conservative wing of the party determined to press ahead with changing the constitution - and also enough MPs in parliament to give it a chance of getting passed.

Mr Abe appears to believe constitutional revision is one of the best hopes his party has of shoring up its vote.

"Prime Minister Abe comes from the wing of the Liberal Democratic Party that has always regarded this document as not being appropriate to Japan, and as being something of an embarrassment," says Professor Deans.

"So partly this move towards constitutional revision has a historical route, partly it is Abe looking for a cause, something to believe in, looking for something to sell to the public. Reform of the constitution is his big ticket item."

For some Japanese, though, there is real concern that the prime minister is trying to abandon the country's pacifist stance.

The constitution stops Japan from doing anything about international issues, so I believe the constitution should be reformed

Toshio Tagima

What is interesting when you ask people their views about constitutional revision is that how old they are tends to affect what they think about the issue.

Eiko Miyanabe, who was born before the end of World War II, does not want to see the document altered.

"For people like myself who are past the mid-70s, we want to shout out our opposition to any change, especially to Article 9 in the constitution," she says.

"My son is now 47 years old and I don't think he would have to go to war but my grandchildren might have to."

Toshio Tagima, a businessman, was born after Japan's defeat in 1945.

"When we were at school, we all thought there was something very odd about the Japanese constitution," he says.

"Right now the constitution stops Japan from doing anything about international issues, so I believe the constitution should be reformed."

'Leave us alone'

The reality is that changing the constitution in parliament, or the Diet as it is known, will not be easy even if the LDP were to strengthen its hold on the upper house in next month's elections.

Professor Masatoshi Honda, from Japan's National Graduate Institute for Policy Studies, points out that any changes have to be approved by both parliament and people, and that under laws passed recently the referendum cannot take place before 2010.

And what about Japan's neighbours? Politicians in China and South Korea often criticise elements in Japan that they believe glorify the country's former militarism.

Prof Honda says the process should be allowed to get underway without any interference from Japan's neighbours, who might fear it signals a return to the country's violent past.

"We don't need any pressure, we don't want to care so much about international circumstances, we don't want to care so much about US-Japan relations," he says.

"The message to outsiders, if you like, is 'leave us alone, we will choose the right way'."

So will China and South Korea trust Japan's leaders enough to get on with it? So far they have said little, but that is perhaps because it is only recently that Mr Abe has hinted at what exactly will be changed.

He has told reporters he will start by revising the constitution item by item, rather than amending it as a whole, and incorporate environmental rights into the document before tackling the preamble and the war-renouncing Article 9.

That cautious approach could blunt criticism from abroad in the early stages of any constitutional revision, but any change to Article 9 will no doubt attract a lot more attention.